1. The Field of the Invention
The invention relates to the use of a glass for glass-metal bonds. The invention also relates to the composition of the glass itself.
2. The Description of the Related Art
Glass-metal bonds are used, for example, in vacuum tube collectors. In vacuum tube collectors, a vacuum-tight glass-metal bond for thermal insulation is required between the absorber tube consisting of metal and the envelope tube consisting of glass.
Such tube collectors are used, for example, in parabolic trough solar power plants.
In parabolic trough solar power plants, the concentrated radiation produces operating temperatures of up to 400° C. in the collectors, with strong local temperature differences. Besides continual stresses due to temperature change are caused by the diurnal cycle and intermittent cloudy periods.
In high-temperature solar collectors, on the one hand so-called unmatched glass-metal bonds have to date been used. Their name is based on the fact that the glass and metal have different coefficients of thermal expansion. On the other hand so-called matched glass-metal bonds are used, in which case one possible way of achieving the match is to use a plurality of intermediate glasses. This approach, however, has various disadvantages.
DE 10 2004 008 559 A1 discloses a glass-metal bond of a tube collector, which uses an aluminum borosilicate glass that is also employed as a material for primary pharmaceutical packaging. It is not specially adapted to the requirements of a glass for tube collectors and to the requirements of producing such a glass. A similar glass composition range of a glass, in particular for solar thermal applications, is described in DE 10 2006 041 469 B3. Both glasses, with relatively high levels of boron oxide and aluminum oxide, are disadvantageous for reasons of cost and environmental protection. DE 44 30 710 C1 discloses low-boron borosilicate glasses which have a coefficient of thermal expansion α20/300=5.3×10−6/K.
The receiver, as the key part of a tube collector for a parabolic trough solar power plant, consists of an inner tube made of steel and an outer envelope tube made of glass. The two components must be connected vacuum-tightly by means of glass-metal bonds. Typical metals of the glass-metal bond are KOVAR® alloys.
The envelope tube requires a coefficient of thermal expansion matched to the metal.
The steel tube (absorber tube) is coated with a radiation-selective material, in order to ensure a high absorption ratio.
The vacuum between the envelope tube and the absorber tube reduces the heat loss and makes a contribution to optimal heating efficiency.
The envelope tube should be composed of a glass that has an index of refraction, which is significantly lower than that of soda-lime glasses, i.e. lower than 1.52, preferably <1.51. In this way the fraction of reflected light is reduced.
The envelope tube is provided with an antireflection layer which, together with the requisite high transmission of the glass, is intended to give a high transmission of at least 96%.
The envelope tube requires high thermal and temperature change stability, high weather resistance, i.e. a high hydrolytic stability, and high mechanical stability, i.e. high strength.
Despite its high load-bearing capacity, in particular thermal load-bearing capacity, it should readily melt in the least energy-intensive way possible. All these glass and production properties should furthermore be achievable with the least possible production and raw material costs.